1. Field of the Invention
This invention relates to a novel and improved antiskid control system for motor vehicles, which is operative to prevent the wheels of the motor vehicle from skidding during braking operation of the motor vehicle.
2. Description of the Prior Art
Generally, with an anti-skid control system for motor vehicles, control is effected by means of microcomputers such that hold valves and decay valves comprising electromagnetic valves are opened and closed on the basis of electrical signals representing wheel speeds sensed by wheel speed sensors, thereby increasing, holding or reducing the brake pressure, for the purposes of securing improved steering performance and running stability of the motor vehicle, while at the same time shortening the braking distance.
FIG. 1 of the accompanying drawings illustrates, by way of example, manners in which wheel speed Vw, wheel acceleration and deceleration +Vw, -Vw Pw are varied during the operation of the conventional anti-skid control system, together with hold signal HS and decay signal DS for opening and closing hold valves and decay vlaves.
When the brake apparatus of a running motor vehicle is not operated, the hold valves remain open while the decay valves remain closed, add the brake pressure Pw is not increased; and when the brake apparatus is operated, the brake pressure Pw is increased at timc t1 so that the wheel speed is decreased. A reference wheel speed Vt is set up which is lower by a predetermined amount .DELTA.V than the wheel speed Vw and follows the latter with such a speed difference. More specifically, the reference wheel speed Vt is set up so that when the deceleration (negative acceleration) -Vw of the wheel reaches a predetermined threshold level, say -1G at time t2, the reference wheel speed Vt is thereafter made to linearly decrease with a decerelation gradient of -1G. At time t3 when the deceleration -Vw of the wheel reaches a predetermined maximum value -Gmax, the hold signal HS is interrupted so that the hold valves are closed, thus holding the brake pressure Pw.
Because of the brake pressure Pw being held, the wheel speed Vw is further decreased. At time t4, the wheel speed Vw and the reference wheel speed Vt become equal to each other, and decay signal DS is generated, by which the decay valves are opened so that reduction of the brake pressure Pw is started. As a result of this reduction of the brake pressure Pw, the wheel speed Vw is changed from deceleration to acceleration, at time t.5 when a low peak V1 of the wheel speed Vw occurs. Either at the time t5 or at time t6 when the wheel speed Vw is increased up to the level of a speed Vb that is higher than the low peak by 15% of the difference A between the wheel speed Va at the time t4 when the reduction of the brake pressure is started, and the low-peak speed V1, the decay signal DS is interrupted, and as a result the decay valves are closed so that the reduction of the brake pressure Pw is stopped and thus the brake pressure is held. The wheel speed Vw is further increased and reaches a high peak at time t7; thereupon, the brake pressure Pw is again increased. In this case, the buildup of the brake pressure Pw is effected in such a manner that the brake pressure Pw is alternately increased and held in succession by the fact that the hold signal HS is turned on and off mincingly so that the brake pressure Pw is caused to gradually build up. In this way, the wheel speed Vw is decreased, and at time t8, the mode for reduction of the brake pressure occurs again. The brake pressure increasing, holding and reducing modes are effected in combination as mentioned above, and thus the wheel speed Vw can be controlled so that the vehicle speed can be decreased, while the wheels of the motor vehicle are prevented from being locked.
In the above conventional anti-skid control system, to avoid any adverse influence of unwanted information noise which tends to be generated depending on the road surface condition, the threshold level of deceleration at which the reference wheel speed Vt is changed so as to linearly decrease with a constant gradient, is selected to be say -1G, which is higher than deceleration which tends to occur in a normal situation. That the deceleration of the wheel speed has reached the aforementioned threshold level -1G, is detected; on the basis of the detection, the reference wheel speed Vt is made to decrease with the deceleration gradient of -1G from the time t2 onward; and at the time t4 when the wheel speed Vw and the reference wheel speed Vt become equal to each other, reduction of the brake pressure Pw is initiated. Thus, in the case where the wheel speed Vw is decreased with a deceleration, say -0.7G, lower than the aforementioned threshold level of deceleration -1G as a result of light braking, for example, the reference wheel speed Vt simply follows the wheel speed Vw with the speed difference .DELTA.V and never becomes equal thereto; hence, no point of time when the brake pressure Pw is to be reduced, is detected so that the wheel speed continues decreasing with an increasing deviation from the vehcile speed. Thus, on a road surface with a low coefficient of friction, early-locking of the wheel is liable to be caused.